1. Field of the Invention
This invention relates to a device for observing an object, having a flat reflection surface and an inclined reflection surface having a certain inclination with respect to the flat reflection surface, such as, for example, masks, wafers, etc. to be used in a printer for IC and LSI.
2. Description of the Prior Art
Steps for fabricating semiconductor elements such as transistors and integrated circuit elements (IC, LSI) include printing of fine patterns, bonding of lead wires, and other related works, all of which require highly precise positioning of the wafer or pellet on a predetermined position for printing the patterns thereon. Since the wafers, etc. are in such a micro-structure of the order of a micron or a sub-micron, the precision in positioning such wafers, etc. should be done to that order. However, this kind of work is extremely intricate even to skilled workers, and takes a long period of practice until such high skill is attained.
In the high precision positioning of a micro-structure on a wafer, on which the printing is to be effected, and a fine pattern on the mask, it has so far been the usual practice for an operator observe an object to be positioned through a microscope, and manually displace the object to a predetermined position. In recent years, however, various attempts have been made to perform this kind of positioning operation by an automatically operated positioning machine. The so-called "automatic aligner" is one type of such machine.
In such automatic alignment, i.e., automatic position adjusting method, a photo-electric expedient is used for the positioning. For example, when a separate pattern is to be superposed and printed on a pattern of an actual circuit element which has already been formed on the wafer, an alignment mark previously provided on the wafer and another alignment mark provided on the mask are brought into coincidence, whereby the formed pattern and the separate pattern are brought to a desired relationship. In this alignment technique, when the alignment mark provided on the wafer and the alignment mark provided on the mask are mutually off their predetermined position, such positional discrepancy is detected photoelectrically to cause a servo-mechanism to operate until the discrepancy becomes nil, thereby adjusting the positional discrepancy between the wafer and the mask. It should be noted that an exclusive alignment mark need not always be provided on the wafer, but a part of the actual element pattern formed thereon may be used as the alignment mark for the same result.
In the above-described photoelectric detection for the alignment technique, the image contrast of the object to be observed constitutes a factor to give a decisive influence on the detection precision. A problem inherent in the conventional automatic alignment device resides in its inability to obtain sufficient image contrast, possible causes for which may be as follows.
First of all, setting aside the quality of the optical system per se, a problem arising from the mask and a problem arising from the wafer are the principal causes. That is to say, reflected light from the surface of chromium, which has been conventionally used as the material for the mask, constitutes a component of flare to deteriorate the SN ratio of a signal, and reflected light from the surface of silicon dioxide or photo-resist coated on the silicon substrate functions in the same way. Besides the above, since the silicon dioxide layer or the photo-resist layer possesses the function of an interference thin film, the pattern to be observed may on some occasion give rise to a reversing phenomenon between a bright portion and a dark portion, when the coated layer is in a certain specific film thickness, to constitute a factor adversely affecting treatment of the pattern portions as an electrical signal.
A device which solved such drawback is disclosed in U.S. Pat. No. 3,796,497 issued Mar. 12, 1974. This patented device utilizes a wafer and a mask, wherein an alignment mark is formed by a flat reflection surface and an inclined reflection surface having an inclination with respect to the flat reflection surface, and disposes a telecentric lens (although the specification does not say specifically the use of the telecentric lens) in such a manner that the optical axis thereof may be orthogonal to the flat reflection surface of the wafer and the mask. At the position where the optical axis of the telecentric lens intersects with the pupil surface thereof, there is formed a light source image which is smaller in size than the pupil opening. By the light from this light source image, the wafer and the mask are illuminated through the telecentric lens. The telecentric lens, when a light source is provided at the intersection of the optical axis and the pupil surface, possesses such a property that principal light becomes parallel with the optical axis. On account of this, the principal light of the light beam from the telecentric lens is projected perpendicularly onto the flat surface of the mask and wafer. The light which has been projected perpendicularly onto the flat surface quite naturally travels backward along the incident light path, and again forms the light source image at the abovementioned position of the light source image. On the other hand, since the light from the inclined reflection surface having an inclination with respect to this flat surface travels with a certain inclination with respect to the optical axis of the telecentric lens, no light source image is formed on the abovementioned position of the light source image. On account of this, the reflected light from the flat surface is removed by arranging a light intercepting means of the same size as that of this light source image at the position of the light source image. Therefore, when the surfaces of the mask and wafer are observed from the backside of this light intercepting means, the flat surface cannot be observed, but only the inclined surface can be observed, as the result of which an alignment mark of very high image contrast can be obtained.
In addition to the afore-described construction, this patented device further provides a semi-transparent mirror on the pupil surface of the telecentric lens at an inclined angle to form the light source image thereon, and, at the same time, arranges the abovementioned filter means on this mirror surface. This arrangement, however, possesses various disadvantages. For example, when the pupil surface of the telecentric lens is not so much separated from the telecentric lens, the distance between the telecentric lens and the position of the pupil surface is normally very short, and, in an extreme case, the pupil surface possibly invades into the lens system. In such case, it is almost impossible to provide the mirror within such small space at an inclined angle. Considering this point, therefore, the lens used in this patented device may not be the telecentric lens.